April 16, 1917. On the ground in France, it appears that the German army has retreated, but Allied reconnaissance planes have reported to a British battalion that it is in fact a tactical withdrawal. Since the Germans have cut the telephone lines to the British trenches, Lance Corporals Will Schofield and Tom Blake are given instructions by General Erinmore that they must deliver a message to Colonel Mackenzie of the Devonshire Regiment's Second Battalion by hand; the message will tell the Colonel that the planned attack is to be called off because it will lead to many casualties. Heightening the stakes is the fact that Tom's brother, Joseph Blake, is among the men fighting in the Second Battalion.
No-man's land, the gap between British and German trenches that belongs to neither side, stands between the soldiers and the abandoned German trenches. Schofield is almost killed when a booby trap goes off in the abandoned barracks, but Blake saves him. They then see a German plane being shot down in a dogfight with an Allied plane. When they try to save the pilot, he stabs Blake. Schofield tries to administer aid to Blake, and makes a promise to complete their mission and find his brother. Schofield takes Blake's dog tags to give to his family after Blake dies.
Schofield crosses what is left of the bridge at Écoust-Saint-Mein, a small bombed-out village. After getting shot at by a sniper, Schofield manages to kill the sniper and rest for a moment in a basement, where he meets a French woman who is hiding from German troops. He then runs through German territory and jumps into a river, which carries him over a waterfall. At the bottom, he climbs to shore and finds himself in the forest where the Second Battalion is preparing for the attack that Erinmore is trying to call off. Schofield runs to deliver the letter, racing across the battlefield and barging into a tactical meeting that Mackenzie is presiding over. Mackenzie reads the message and calls off the attack.
Schofield goes to the front of the British line and finds Joseph Blake to tell him that his brother has been killed. He gives Joseph the dog tags he had taken from Blake and sits down to write a letter to Blake's mother, as he had promised. Before he begins to write, he takes out photographs of his wife and two little girls who are waiting for him at home.